The present invention relates to a belt-type fixing device that is used in an electrophotographic image forming apparatus.
In Japanese Patent Laid-Open Publication 2001-356625, a fixing device has been disclosed in which pressure contact of a pressurizing pad with a thermal fixing roller having a heat source therein through an endless belt forms a fixing nip between the thermal fixing roller and the endless belt. In the fixing device, the thermal fixing roller has an elastic layer on an outer circumference thereof, and the elastic layer is pressed by the pressurizing pad through the endless belt so as to be strained. The fixing device is configured so that nip pressures in the fixing nip is larger in vicinity of an exit for a recording sheet.
Such a variation in the nip pressure in the fixing nip, however, results in a variation in amount of strain of the elastic layer on the thermal fixing roller. Such a difference in amount of strain of the elastic layer on the thermal fixing roller with respect to a sheet conveying direction in the fixing nip leads to a little difference in sheet conveying velocity among areas in the fixing nip with respect to the sheet conveying direction. The difference in conveying velocity in the fixing nip causes a problem in that the difference stresses a sheet passing through the nip and causes image noise such as image blur, wrinkles of paper or the like.